The field of the invention comprises photoconductive materials and principally their application to recording and imaging. More specifically the invention is concerned with a novel method of depositing a photoconductive coating that has a wide range of uses because of its ability to function as means for storing and selectively giving up electrical charge.
One very important use of photoconductive materials is in the art of electrophotography where electrostatic images are made and developed. The field of electrophotographic members and their construction and use is also involved herein.
The background of the invention is, by reason of the wide utility of the coating thereof, concerned with work that has been carried out in many disciplines. The most familiar of these is the art of electrostatics where an electrophotographic member is charged uniformly in darkness, exposed by projecting light from an object onto the charged member to dissipate or discharge the electrical charge selectively, developed by applying charged particles to the resulting latent electrostatic image of the object and further processed. The further processing is for the purpose of preserving the developed image of the object, either by transfer of the developed image to a carrier or by fixing the developed image directly onto the electrophotographic member.
Both of the processes described above are well known respectively as xerography and electrofax, and the art on the subject is voluminous and need not be detailed. The basic techniques, that is, charging, exposing, developing and further processing are capable of being performed with electrophotographic members of the invention having as a part thereof the coating of the invention. The electrophotographic member of the invention differs from known electrophotographic members primarily in that its composition is different and its performance is vastly superior to that of known electrophotographic members.
The electrophotographic member of the invention, among its other attributes differs from the prior art by reason of its high speed, high field strength, high gain, ability to achieve infinite grey scale, absence of residual charge upon discharge, absence of fatigue, low cost, durability, high resolution and panchromaticity. When deposited on a polyester or other stable plastic sheeting substrate it is highly flexible and transparent to a degree that images formed thereon can be projected.
Treated in the same manner as photographic film, that is, using the same conditions of light and exposure the electrophotographic film of the invention is superior to photographic film because it is archival in nature, has higher resolution, images can be added to it, and the film can be reused if not processed after development. It is made out of easily obtained materials in ambient light and can be handled and stored without the precautions required for silver halide film. It is made without using the scarce metal, silver, in its manufacture.
The invention is also useful for almost any field that a photoconductive material can be used, and is advantageous because of its high quantum gain which is greater than unity, its anisotropy, its flexibility (when deposited on a flexible substrate), its transparency.
The principal material from which the photoconductive coating of the invention is made is cadmium sulfide, deposited in a sputtering process using R.F. energy. Cadmium sulfide has been known as a photoconductive material for many years and there is a vast body of literature which is concerned with work done on this and similar materials. So far as known, no prior art teachings have succeeded in achieving the coating of the invention with all of its advantages and benefits, as detailed in the specification. Further, no use has been made, so far as known, of a coating of the properties of the invention in an electrophotographic member.
Some of the prior art photoconductive coatings which are used especially in electrophotography are selenium, zinc oxide and polyvinyl carbasol. Selenium is used in an amorphous form, is soft and easily abraded. It requires high voltages for charging and when changed provides surface voltages that are of the order of 600 volts, that is, about twenty times those which are achieved on the surface of the coating of the electrophotographic member of the invention. Notwithstanding this, the field strength represented by volts per centimeter of thickness is substantially less than those achieved by the invention. Selenium has a noise voltage which is higher than the surface potential of the charged coating of the invention. It has a residual charge, it fatigues, it is slow because its gain is very low. Zinc oxide is used in a binder matrix. It is applied to a paper which is conductive and also has low speed. It is soft, incapable of being imaged with high resolution images, is practically opaque when used. Polyvinyl carbasol is an organic material subject to all of the vagaries of organic compounds, is slow and unstable, soft, not transparent, has limited spectral response, etc.